1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine and a printer, in which an electrophotographic system or an electrostatic recording system is employed.
2. Description of the Related Art
Conventionally, there is provided an image forming apparatus using a two-component developer. The two-component developer includes toner and carriers, of which a TD ratio representing a ratio of the toner to the carriers changes as the toner is consumed when image forming is progressed. As a result, toner charging characteristics vary, and thus an image density varies. For this reason, the toner is replenished in order to suppress the variation in the image density.
As a method for the toner replenishment control, there is a method of stabilizing the toner charging characteristics, which is called a patch detection ATR control (see Japanese Patent Laid-Open No. 10-039608). In the method, an image pattern (a patch image) for detecting the image density is formed on an electrophotographic photoreceptor. Then, the image density of the patch image is detected by an image density sensor, and a toner replenishment amount is controlled such that the detected value becomes a desired value.
However, in the patch detection ATR control, there may be a case in which an appropriate developer state is not maintained when the development characteristic of the patch image varies. For example, even in a case where the toner charging characteristics are not changed, the image density of the patch image becomes pale when the development characteristic is degraded. In this case, the patch detection ATR control is performed to increase the TD ratio, so that the image density of the patch image becomes a desired value. As a result, there is a concern that the toner charging characteristics are deteriorated, and the degradation in image quality such as granularity (roughness), fogging on a white base, and a void image due to transfer defects, and the toner scattering may occur.
For this reason, the TD ratio detecting unit such as an inductance sensor is provided in a developing device, and a limit is set in the TD ratio (see Japanese Patent Laid-Open No. 2007-78896). Accordingly, the degradation in the toner charging characteristics can be suppressed, and the image degradation and the toner scattering can be suppressed.
However, according to Japanese Patent Laid-Open No. 2007-78896, the inductance sensor is necessarily provided, and thus the cost is incurred.